The ability of neoplastic cells to initiate DNA synthesis and proliferate in Ca2+-deficient medium distinguishes them from their non-neoplastic counterparts. A clue to understanding this alteration is the fact that tumor promoting agents enable non-neoplastic rat liver epithelial cells to initiate DNA synthesis and proliferate in Ca2+-deficient medium like their neoplastic counterparts. It follows that uncovering the mechanisms responsible for this phenomenon should not only enhance our knowledge of the neoplastic process but also provide opportunities to modulate the tumor promotion process. These are our long range objectives. This proposal will test the hypothesis that tumor promoters in general stimulate Ca2+-deprived rat liver cells to initiate DNA synthesis through a mechanism involving Ca2+-calmodulin, cyclic AMP, cyclic AMP-dependent protein kinases and Ca2+/phospholipid-dependent protein kinase activity. The cell surface and intracellular phosphoprotein substrates will be identified and their requirement for DNA-synthetic activity and tumor promotion determined.